You do not have to purchase the inspection devices. But depending upon local codes, you may have to pay a certified plumber to inspect/test your backflow.
As a mater of fact, if you have not yet, the 1st thing you need to do is check with local building code. Legal requirements range from "do what ever you want... you are liable for your own actions" to "only a certified plumber is allowed to install irrigation".
Typical legal requirements are usually one of the following:
You must install a backflow preventer (no legal requirements as to which , to test, or inspect).
You must install one of these backflow preventers ___, ____, ____
(the list can be as short as specifying a particular part number)
You must install a backflow preventer and have it tested by a certified plumber annually.
A certified plumber is required to install all irrigation lines from the mainline to the backflow preventer.